On exposure to acid, pH, swine pepsinogen undergoes self cleavage to produce psuedo pepsin, by the loss of the first 16 residues in its sequence. Returning to neutrality, produces an irreversible conformational change in psuedo pepsin to a new globular form, which seems to be identical to the rapidly formed transient intermediate previously detected during the folding of intact pepsinogen. Like pepsinogen, but unlike native psuedo pepsin, this neutral pH form of the protein can be reversibly unfolded by urea or high pH. The mechanism of the folding reaction, the structure and stability of this neutral pH form of psuedo pepsin and the role of the first 16 residues in the mechanism of folding of pepsinogen are all under investigation.